Sensor clip assembly for an optical monitoring system

ABSTRACT

Systems and sensor clip assemblies for optically monitoring blood flowing through a blood chamber are provided. A sensor clip assembly includes emitters and photodetectors positioned on opposing arms, a signal conditioning circuit for conditioning raw analog signals generated by the photodetectors while the sensor clip assembly is fastened to a blood chamber, and an analog-to-digital converter for converting the conditioned analog signals to raw digital data. The sensor clip assembly may output the raw digital data to an external device and receive synchronized control signals from the external device, or the sensor clip assembly may include a microcontroller for performing calculations on the raw digital data and providing synchronized control signals internally. Parameters of blood flowing through the blood chamber such as hematocrit, oxygen saturation, and change in blood volume may be calculated from the raw digital data derived from the raw analog signals generated by the photodetectors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of copending U.S. patentapplication Ser. No. 13/299,303, filed Nov. 17, 2011, which is acontinuation-in-part of copending U.S. patent application Ser. No.13/030,212, filed Feb. 18, 2011, which claims the benefit of U.S.Provisional Patent Application No. 61/414,654, filed Nov. 17, 2010. U.S.patent application Ser. No. 13/299,303 claims priority to U.S.Provisional Application No. 61/553,078, filed Oct. 28, 2011. U.S. patentapplication Ser. No. 13/299,303 is also a continuation-in-part of U.S.application Ser. No. 13/034,788, filed Feb. 25, 2011. All of theforegoing patent applications are incorporated by reference herein intheir entireties.

FIELD

The invention relates to optical monitoring systems, and morespecifically systems for monitoring the presence or concentration ofconstituents in blood. The invention is particularly useful for thereal-time measurement of hematocrit and/or oxygen saturation levels whenmonitoring a patient during hemodialysis or other procedure involvingextracorporeal blood flow.

BACKGROUND

Patients with kidney failure or partial kidney failure typically undergohemodialysis treatment in order to remove toxins and excess fluids fromtheir blood. To do this, blood is taken from a patient through an intakeneedle or catheter which draws blood from an artery or vein located in aspecifically accepted access location—e.g., a shunt surgically placed inan arm, thigh, subclavian and the like. The needle or catheter isconnected to extracorporeal tubing that is fed to a peristaltic pump andthen to a dialyzer that cleans the blood and removes excess fluid. Thecleaned blood is then returned to the patient through additionalextracorporeal tubing and another needle or catheter. Sometimes, aheparin drip is located in the hemodialysis loop to prevent the bloodfrom coagulating.

As the drawn blood passes through the dialyzer, it travels in straw-liketubes within the dialyzer that serve as semi-permeable passageways forthe unclean blood. Fresh dialysate solution enters the dialyzer at itsdownstream end. The dialysate surrounds the straw-like tubes and flowsthrough the dialyzer in the opposite direction of the blood flowingthrough the tubes. Fresh dialysate collects toxins passing through thestraw-like tubes by diffusion and excess fluids in the blood by ultrafiltration. Dialysate containing the removed toxins and excess fluids isdisposed of as waste. The red cells remain in the straw-like tubes andtheir volume count is unaffected by the process.

An optical blood monitoring system is often used during hemodialysistreatment or other treatments involving extracorporeal blood flow. Oneexample is the CRIT-LINE® monitoring system sold by Fresenius USAManufacturing, Inc. of Waltham, Mass. The CRIT-LINE® blood monitoringsystem uses optical techniques to non-invasively measure in real-timethe hematocrit and the oxygen saturation level of blood flowing throughthe hemodialysis system. The blood monitoring system measures the bloodat a sterile blood chamber attached in-line to the extracorporealtubing.

In general, blood chambers along with the tube set and dialyzer arereplaced for each patient. The blood chamber is intended for a singleuse. The blood chamber defines an internal blood flow cavity comprisinga substantially flat viewing region and two opposing viewing lenses. LEDemitters and photodetectors for the optical blood monitor are fastened(e.g., by clipping) into place onto the blood chamber over the lenses.Multiple wavelengths of light may be resolved through the blood chamberand the patient's blood flowing through the chamber with a photodetectordetecting the resulting intensity of each wavelength.

The preferred wavelengths to measure hematocrit are about 810 nm, whichis substantially isobestic for red blood cells, and about 1300 nm, whichis substantially isobestic for water. A ratiometric techniqueimplemented in the CRIT-LINE® controller, substantially as disclosed inU.S. Pat. No. 5,372,136 entitled “System and Method for Non-InvasiveHematocrit Monitoring,” which issued on Dec. 13, 1999 and is assigned tothe assignee of the present application, uses this light intensityinformation to calculate the patient's hematocrit value in real-time.The hematocrit value, as is widely used in the art, is a percentagedetermined by the ratio between (1) the volume of the red blood cells ina given whole blood sample and (2) the overall volume of the bloodsample.

In a clinical setting, the actual percentage change in blood volumeoccurring during hemodialysis can be determined, in real-time, from thechange in the measured hematocrit. Thus, an optical blood monitor isable to non-invasively monitor not only the patient's hematocrit levelbut also the change in the patient's blood volume in real-time during ahemodialysis treatment session. The ability to monitor real-time changein blood volume helps facilitate safe, effective hemodialysis.

To monitor blood in real time, light emitting diodes (LEDs) andphotodetectors for them are mounted on two opposing heads of a sensorclip assembly that fit over the blood chamber. For accuracy of thesystem, it is important that the LEDs and the photodetectors be locatedin a predetermined position and orientation each time the sensor clipassembly is clipped into place over the blood chamber. The predeterminedposition and orientation ensures that light traveling from the LEDs tothe photodetectors travels through the lenses of the blood chamber.

The optical monitor is calibrated for the specific dimensions of theblood chamber and the specific position and orientation of the sensorclip assembly with respect to the blood chamber. For this purpose, theheads of the sensor clips are designed to mate to the blood chamber sothat the LEDs and the photodetectors are at a known position andorientation. In the CRIT-LINE® monitoring system, the head of the sensorclips and the blood chamber have complementary D-shaped configurations.

In conventional systems, the optical monitoring is performed by astand-alone controller that includes a display that presents themonitoring data in real-time. The controller includes a processor thatcalculates the displayed data and controls the operation of the LEDs andphotodetectors. The controller is conventionally connected to the sensorclip and the optical devices via a tethering cable. A significant amountof noise is introduced to the analog signal provided by thephotodetectors during transmission through a cable to the stand-alonecontroller, and the amount of power required to illuminate the LEDs tocompensate and ensure a useable analog signal generates heat whichdegrades the lifetime of the LEDs. Furthermore, photodiode currents areso small that any series resistance in its connection is an attenuatorand potential noise source. The longer the cable for the analog signal,the more resistance there is to the current and the more noise therewill be in the signal.

SUMMARY

In an embodiment of the present invention, a sensor clip assembly foroptically monitoring blood flowing through a blood chamber is provided.The sensor clip assembly includes: a housing having two opposing armscapable of being fastened to a blood chamber; at least one emitter inone of the opposing ends; at least one photodetector in the otheropposing end positioned relative to the at least one emitter such thatlight emitted by the at least one emitter is capable of being receivedat the at least one photodetector after passing through a blood chamberto which the sensor clip assembly is fastened; a microcontroller withinthe housing configured to receive conditioned analog signals, whereinthe conditioned analog signals are based on raw analog signals generatedby the at least one photodetector, to convert the conditioned analogsignals to raw digital data, and to calculate at least one parametercorresponding to blood in a blood chamber to which the sensor clipassembly is fastened based on the raw digital data; and an output portconfigured to output from the sensor clip assembly results ofcalculations performed by the microcontroller to an external device.

The sensor clip assembly may further include at least one transimpedenceamplifier within the housing corresponding to each photodetector forconverting raw analog signals to analog voltage signals; and at leastone digitally-controllable trimpot within the housing corresponding toeach photodetector for applying a gain to the analog voltage signals.The microcontroller may be further configured to control operation ofthe at least one emitter, and to control the gain applied by the atleast one digitally-controllable trimpot in a manner that issynchronized with the operation of the at least one emitter. At leastone of the emitter arm and the photodetector arm may include a shroudfor blocking ambient light from being received at the at least onephotodetector.

The sensor clip assembly may further include a silicon photodetector andan Indium-Gallium-Arsenide photodetector, and the microcontroller mayfurther be configured to calculate a hematocrit value, an oxygensaturation value, and a percent blood volume change. The output port ofthe sensor clip assembly may correspond to a USB (Universal Serial Bus)connection, and the external device may be a computer. The output portmay further be configured to transmit commands received from theexternal device to the microcontroller. Further, the microcontroller maybe configured to verify the accuracy of the sensor clip assembly basedon a unique verification filter, and to recalibrate the sensor clipassembly upon confirming user input of a correct verification filteridentification code. The microcontroller may further be part of a boardfloated within one of the two opposing arms.

In another embodiment, a system for optically monitoring blood isprovided. The system includes: a blood chamber comprising a viewingwindow and a chamber body; a sensor clip assembly fastened to the bloodchamber, the sensor clip further including a housing having an emitterarm and a photodetector arm, at least one emitter within the emitterarm, at least one photodetector within the photodetector arm positionedrelative to the at least one emitter such that light emitted by the atleast one emitter is capable of being received at the at least onephotodetector after passing through the blood chamber, a microcontrollerwithin the housing configured to receive conditioned analog signals,wherein the conditioned analog signals are based on raw analog signalsgenerated by the at least one photodetector, to convert the conditionedanalog signals to raw digital data, and to calculate at least oneparameter corresponding to blood in a blood chamber to which the sensorclip assembly is fastened based on the raw digital data, and an outputport configured to output results of calculations performed by themicrocontroller from the sensor clip assembly to an external device; andthe external device, configured to display the results of thecalculations performed by the microcontroller to a user.

The emitter arm and the photodetector arm may further be opposing armsbiased together at first opposing ends of the arms to form a jaw suchthat a pinching force applied to second opposing ends of the arms opensthe jaw to allow the blood chamber to placed between the first opposingends and held there when the force is removed. The chamber body of theblood chamber may be tinted blue so as to block ambient light from beingreceived at the at least one photodetector. The system may furtherinclude a verification filter uniquely associated with the sensor clipassembly for determining whether recalibration of the sensor clipassembly is needed. The output port may be further configured totransmit commands received from the external device to themicrocontroller; and the microcontroller may be further configured toverify accuracy of the sensor clip assembly based on the verificationfilter and to recalibrate the sensor clip assembly upon confirming userinput of a correct verification filter identification code. Themicrocontroller may further be part of a board floated within one of theemitter arm and the photodetector arm.

In yet another embodiment, a sensor clip assembly having amicrocontroller, an emitter, and a photodetector, with themicrocontroller further including a processor and a tangible,non-transient computer-readable medium having computer-executableinstructions for optically monitoring blood stored thereon is provided.The computer-executable instructions include: instructions for turningthe emitter on, wherein the emitter corresponds to the photodetector;instructions for synchronizing conditioning of raw analog signalsgenerated by the photodetector on a channel corresponding to thephotodetector with operation of the emitter; instructions forcalculating at least one parameter corresponding to blood based on rawdigital data converted from conditioned analog signals, wherein theconditioned analog signals are based on the raw analog signals generatedby the photodetector; and instructions for outputting results ofcalculations to an external device via an output port.

The computer-executable instructions may further include instructionsfor controlling an amount of gain applied by a digitally-controllabletrimpot on the channel corresponding to the photodetector, instructionsfor verifying accuracy of the sensor clip assembly based on averification filter uniquely associated with the sensor clip assemblyupon receiving a corresponding command from the external device,instructions for receiving a user input of a verification filteridentification code; instructions for recalibrating the sensor clipassembly if the verification filter identification code input by theuser corresponds to the verification filter uniquely associated with thesensor clip assembly, and/or instructions for outputting statusinformation corresponding to the sensor clip assembly to the externaldevice. The computer-executable instructions for outputting results ofcalculations to an external device via an output port may furtherinclude instructions for outputting a data stream including informationpertaining to a hematocrit value, an oxygen saturation value, and apercent blood volume change.

In yet another embodiment, a sensor clip assembly for opticallymonitoring blood flowing through a blood chamber is provided. The sensorclip assembly includes: a housing having two opposing arms capable ofbeing fastened to a blood chamber; means for fastening the housing tothe blood chamber; at least one emitter in one of the opposing ends; atleast one photodetector in the other opposing end positioned relative tothe at least one emitter such that light emitted by the at least oneemitter is capable of being received at the at least one photodetectorafter passing through a blood chamber to which the sensor clip assemblyis fastened; a signaling conditioning circuit configured to apply a gainto and to filter noise from raw analog signals generated by the at leastone photodetector; an analog-to-digital converter configured to convertconditioned analog signals to raw digital data; and an output portconfigured to connect the sensor clip assembly to an external device.

The signaling conditioning circuit may further include at least onetransimpedence amplifier, at least one digital trimpot, and a filtercircuit. At least one of the two opposing arms may include a shroud forblocking ambient light from being received at the at least onephotodetector. In a further embodiment, the sensor clip assemblyincludes a microcontroller within the housing configured to calculate atleast one parameter corresponding to blood in a blood chamber to whichthe sensor clip assembly is fastened based on the raw digital data; andthe output port of the sensor clip assembly is further configured tooutput results of calculations performed by the microcontroller from thesensor clip assembly to the external device. The output port may befurther configured to transmit commands received from the externaldevice to the microcontroller, and the microcontroller may be furtherconfigured to verify accuracy of the sensor clip assembly based on averification filter uniquely associated with the sensor clip assemblyupon receiving a corresponding command from the external device. Themicrocontroller may be further configured to recalibrate the sensor clipassembly upon confirming user input of a correct verification filteridentification code. The microcontroller may further be part of a boardfloated within one of the two opposing arms.

In yet another embodiment, a system for optically monitoring blood isprovided. The system includes: a blood chamber comprising a viewingwindow and a chamber body; a sensor clip assembly fastened to the bloodchamber, the sensor clip assembly a housing having an emitter arm and aphotodetector arm, at least one emitter within the emitter arm, at leastone photodetector within the photodetector arm positioned relative tothe at least one emitter such that light emitted by the at least oneemitter is capable of being received at the at least one photodetectorafter passing through the blood chamber, a signaling conditioningcircuit configured to apply a gain to and to filter noise from rawanalog signals generated by the at least one photodetector, ananalog-to-digital converter configured to convert conditioned voltageanalog signals to raw digital data, and an output port configured toconnect the sensor clip assembly to an external device; and the externaldevice, configured to receive data from the sensor clip assembly via theoutput port.

The signaling conditioning circuit may further include at least onetransimpedence amplifier, at least one digital trimpot, and a filtercircuit. At least one of the photodetector arm and the emitter arm mayinclude a shroud for blocking ambient light from being received at theat least one photodetector. The chamber body of the blood chamber may betinted blue so as to block ambient light from being received at the atleast one photodetector.

In one further embodiment, the sensor clip assembly further includes amicrocontroller within the housing configured to calculate at least oneparameter corresponding to blood in a blood chamber to which the sensorclip assembly is fastened based on the raw digital data, and the outputport of the sensor clip assembly is further configured to output resultsof calculations performed by the microcontroller from the sensor clipassembly to the external device. The system may further include averification filter uniquely associated with the sensor clip assemblyfor determining whether recalibration of the sensor clip assembly isneeded. The output port may be further configured to transmit commandsreceived from the external device to the microcontroller, and themicrocontroller may be further configured to verify accuracy of thesensor clip assembly based on the verification filter and to recalibratethe sensor clip assembly upon confirming user input of a correctverification filter identification code. The microcontroller may furtherbe part of a board floated within one of the emitter arm and thephotodetector arm.

In an alternative further embodiment, the external device is furtherconfigured to receive the raw digital data from the sensor clip assemblyvia the output port and to calculate at least one parametercorresponding to blood in a blood chamber to which the sensor clipassembly is fastened based on the digital data. The external device maybe further configured to verify accuracy of the sensor clip assemblybased on a verification filter uniquely associated with the sensor clipassembly, and to recalibrate the sensor clip assembly upon confirminguser input of a correct verification filter identification code.

In yet another embodiment, a computing device connected to a sensor clipassembly having an emitter, a photodetector, a signal conditioningcircuit, and an analog-to-digital converter is provided. The computingdevice includes a processor and a tangible, non-transientcomputer-readable medium having computer-executable instructions foroptically monitoring blood stored thereon. The computer-executableinstructions include: instructions for turning the emitter on, whereinthe emitter corresponds to the photodetector; instructions forsynchronizing operation of the signal conditioning circuit withoperation of the emitter corresponding to the photodetector;instructions for receiving, from the sensor clip assembly, raw digitaldata converted by the analog-to-digital converter from conditionedanalog signals based on raw analog signals generated by thephotodetector; and instructions for calculating at least one parametercorresponding to blood based on the raw digital data.

The computer-executable instructions may further include instructionsfor controlling an amount of gain applied by the signal conditioningcircuit, instructions for verifying accuracy of the sensor clip assemblybased on a verification filter uniquely associated with the sensor clipassembly, instructions for receiving a user input of a verificationfilter identification code, and/or instructions for recalibrating thesensor clip assembly if the verification filter identification codeinput by the user corresponds to the verification filter uniquelyassociated with the sensor clip assembly. The computer-executableinstructions for calculating at least one parameter corresponding toblood based on the raw digital data may further include instructions forcalculating a hematocrit value, an oxygen saturation value, and apercent blood volume change corresponding to the blood.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block diagram of an exemplary environment depicting apatient undergoing hemodialysis treatment.

FIG. 2 is a perspective view of a blood chamber.

FIG. 3 is a perspective view of a sensor clip assembly.

FIG. 4 is a perspective view of a sensor clip assembly clipped to ablood chamber.

FIG. 5 is an internal view of a sensor clip assembly depicting internalcomponents of the sensor clip assembly.

FIG. 6 is a schematic diagram of a cross-section of a sensor clipassembly.

FIG. 7 is flowchart depicting a process for collecting, processing, andoutputting data.

FIG. 8 is a block diagram of components of a sensor clip assembly.

FIG. 9 is a functional block diagram of components of a sensor clipassembly according to the embodiment illustrated in FIG. 8.

FIG. 10 is a functional block diagram of components of a sensor clipassembly according to an alternative embodiment.

FIG. 11 is a timing diagram for powering LEDs in the sensor clipassembly and collecting data from complementary sensors.

FIG. 12 is a screen capture of an exemplary demo software interface on acomputer in communication with a sensor clip assembly.

FIGS. 13A-13F are screen captures of an exemplary software interface forcommercial use on an external host device in communication with a sensorclip assembly pertaining to verification and recalibration.

DETAILED DESCRIPTION

An exemplary environment suitable for various implementations of thepresent invention is described with reference to FIG. 1. The exemplaryenvironment 100 of FIG. 1 schematically represents a system where apatient 10 is undergoing hemodialysis treatment with a sensor clipassembly 34 monitoring the patient's blood in real-time as it passesthrough extracorporeal tubing in the hemodialysis system utilizing aconventional blood chamber and sensor clip assembly. It will beappreciated that the described environment is an example and thatcomponents of the environment may be varied or modified withoutdeparting from the teachings contained herein.

An input needle or catheter 16 is inserted into an access site of thepatient 10, such as a shunt in the arm, and is connected toextracorporeal tubing 18 that leads to a peristaltic pump 20, part of ahemodialysis machine 12, and then to a dialyzer or blood filter 22. Thedialyzer 22 removes toxins and excess fluid from the patient's blood.The dialyzed blood is returned from the dialyzer 22 to the patientthrough extracorporeal tubing 24 and a return needle or catheter 26. Theextracorporeal blood flow in the United States generally receives aheparin drip to prevent clotting although that is not shown in FIG. 1.Excess fluids and toxins are removed by clean dialysate liquid which issupplied to the dialyzer 22 via tube 28 and removed for disposal viatube 30. A typical hemodialysis treatment session in the United Statestakes about 3 to 5 hours. In a typical hemodialysis treatment asdescribed in FIG. 1, the access site draws arterial blood from thepatient. If no arterial access is available then a venous catheter maybe used to access the patient's blood. As mentioned, other dialysisapplications such as low flow Continuous Renal Replacement Therapy(CRRT) sometimes used in the Intensive Care Unit or applications such ashigh-flow perfusion measurements during cardiac surgery also measureblood from the patient. Applications include closed-loop blood flowdevices such as conventional dialysis machines, but also may includeapplications with cyclical blood-cleaning devices such as the“single-needle” dialysis technique. Current art indicates that oxygensaturation levels in venous blood correlate to the cardiac output forthe patient.

Optical blood monitoring is performed by the sensor clip assembly 34,which is fastened to a blood chamber 32. While fastening is describedherein with respect to “clipping” via a spring-biased bridge, it will beappreciated that the sensor clip assembly is not required to be a “clip”and may be fastened in a variety of ways, such as through use of aplug-in connector, a snap-in connector, different types of hinges, andother types of fastening mechanisms known to those skilled in the art.Digital data, which may be raw digital data (i.e., representing readingsfrom photodetectors of the sensor clip assembly which have beenconditioned and converted to digital form) or processed digital data(i.e., representing calculations based on the readings from thephotodetectors of the sensor clip assembly), is output from the sensorclip assembly 34 through an appropriate digital processing port such asa USB port. The blood chamber 32 is preferably located in line with theextracorporeal tubing 18 upstream of the dialyzer 22, although it can belocated anywhere in the blood line. Blood from the peristaltic pump 20flows through the tubing 18 into the blood chamber 32. In an embodiment,the sensor clip assembly 34 includes LED photoemitters that emit lightat substantially 810 nm, which is isobestic for red blood cells,substantially 1300 nm, which is isobestic for water, and atsubstantially 660 nm, which is sensitive for oxygenated hemoglobin. Theblood chamber 32 includes windows so that the sensor emitters anddetector(s) can view the blood flowing through the blood chamber 32, anddetermine the patient's real-time hematocrit value and oxygen saturationvalue using known ratiometric techniques. It will be appreciated thatother types of emitters may be used other than LED emitters, such aslaser diodes or a white light source in combination with a prism.

FIGS. 2-4 show the blood chamber 32 and sensor clip assembly 34 in onespecific embodiment. Referring to FIG. 2, the body 301 of the bloodchamber 32 is made of molded, medical grade, blue-tinted polycarbonateor other suitable material. The viewing window 306 on the chamber body301 is preferably made of clear, medical grade polycarbonate materialwhich is molded with a polished finish in order to facilitate reliablelight transmission, e.g. Bayer Makrolon FCR2458-55115 (no regrindallow), which is blood contact approved, USP XX11, Class V1. It isexpected that the material be certified as to grade number, lot numberand date of manufacture.

Although only one side of the blood chamber 32 is depicted by FIG. 2,both sides of the blood chamber 32 include lenses 305 having viewingwindows. As seen in FIG. 2, each lens 305 includes two concentric ringsof ridges, and the inner ring surrounds the viewing window 306 of thelens 305. The outer ring is at the periphery of the lens 305 where thelens mates to the chamber body 301. The annular surface area of the lens305 between the inner and outer rings defines a recess for receiving theshroud of the sensor clip assembly. When mated, the recess and thespring bias of the clip assembly hold the sensor clip assembly 34 andblood chamber 32 together, as will be described in further detail below.To prevent relative rotation of the clip and the blood chamber, a finger307 extends radially inwardly from the ridge of the outer ring. Thisfinger 307 mates to a notch in the shroud and serves to rotationallylock the mated clip assembly and blood chamber. The inlet and outlet ofthe blood chamber 32 are designed to be compatible with standard medicalindustry connecting devices, conventionally known as luer lockconnectors. Alternatively, one or both of the inlet and outlet may beconfigured to include an opening that accepts the outer circumference ofcorresponding tubing. Further detail regarding the configuration anddesign of the blood chamber 32 can be found in U.S. ProvisionalApplication No. 61/553,078, U.S. application Ser. No. 13/034,788, andU.S. application Ser. No. 12/876,572.

FIG. 3 depicts an external view of the sensor clip assembly 34, and FIG.4 provides an example of the sensor clip assembly 34 clipped onto theblood chamber 32. The sensor clip assembly 34 monitors the patient'sblood flowing through the blood chamber 32 (e.g., hematocrit,hemoglobin, change in blood volume and oxygen saturation level, and/orother blood constituents of blood flowing through the blood chamber 32).The casing of the sensor clip assembly 34 includes an LED emitter arm344 and a photodetector arm 346, which are connected via a spring biasedbridge 348. The LED emitter arm 344 contains an emitter subassembly withat least two LED emitters, one emitting infrared light radiation at afirst wavelength (λ₁) of about 1300 nm and another emitting infraredlight radiation at a second wavelength (λ₂) of about 810 nm. The LEDemitter preferably also includes a third LED emitter for emittingvisible light radiation at a third wavelength (λ₃) of about 660 nm.Other wavelengths could be substituted or added to measure additionalblood constituents or properties of other fluids. The detector arm 346contains preferably two types of photodetectors: a silicon photodetector to detect the approximate 660 and 810 nm wavelengths, and anindium gallium arsenide photo detector to detect the approximate 1300 nmwavelength.

The sensor clip assembly 34 further includes two shrouds. One shroud 340is on the inner housing piece of the emitter arm 344 subassembly andprevents ambient light from entering the blood chamber through theviewing windows. A second shroud 342 is on the inner housing piece ofthe detector arm 346 subassembly and also prevents ambient light fromentering the blood chamber through the viewing windows. Shroud 342contains an outer annular ledge or step surface 350 and an inner annularledge or step surface 352. The difference in the heights of the stepsurfaces 350, 352 corresponds to the height of an annular wall on anexterior side of the blood chamber 32 (see FIG. 2), and also to theheight at which a window surface is raised above a sunken well on oneside of the blood chamber 32. Preferably, the shape and surface area ofthe outer annular step surface 350 substantially complements the shapeand surface area of the respective shroud mating surfaces on the bloodchamber 32 in order to maximize the blocking of ambient light. Shroud340 is configured in a similar manner to mate with the opposing exteriorwall of the blood chamber 32. Further detail regarding the structure anddesign of the casing for the sensor clip assembly 34 can be found inU.S. Provisional Application No. 61/553,078, U.S. application Ser. No.13/034,788, and U.S. application Ser. No. 12/876,572.

FIG. 5 depicts an internal view of the sensor clip assembly 34. In FIG.5, the casing for the emitter arm 344 and the detector arm 346 istransparently depicted by the dotted lines. The emitter arm 344 includesan LED circuit board 148 and a transmitter and processor circuit board150. The detector arm 346 of the sensor clip assembly 34 includes adetector circuit board 152, a receiver and communications board 154, anda power supply circuit board 156. A serial cable (e.g., RS-232, USB,etc.) 158 is connected to the receiver and communications circuit board154 and the power supply board 156 on the detector arm. The receiver andcommunications board 154 is connected to the transmitter and processorboard 150, for example, via a pair of seven conductor ribbon cables 160.It will be appreciated that other types of serial cables, such as acable having a NEMA 250 rated bayonet locking connector, may also beused. It will be appreciated that the particular configuration of boardsand connectors depicted in FIG. 5 is merely exemplary. For example, allof the boards could be mounted into one arm or the other (except for theemitters and detectors, which should be mounted on opposing arms), or asin another embodiment described further below, where the sensor clipassembly 34 includes limited circuitry for processing the analog signalsto raw digital data for transmission to an external host device bycable.

FIG. 6 depicts a schematic diagram of a cross-section of the sensor clipassembly 34 clipped to a blood chamber 32. The housing for the sensorclip assembly 34 includes an inner housing frame 162 as well as outerhousing shells 164, 166 for the emitter arm 144 and the detector arm146, respectively. The inner housing frame 162 serves as the innerhousing for both the emitter arm 144 and the detector arm 146. Thebridge 102 spans between the portions of the inner frame housingcorresponding to the emitter arm 144 and the detector arm 146. Thebridge 102 includes an internal channel through which the pair of ribboncables 160 passes. The inner housing frame 162 also includes a springthat spans both arms 144, 146 and the bridge 102 (the spring is notdepicted). The spring biases the distal ends of the emitter arm 144 andthe detector arm 146 towards one another so that they clip securely overthe blood chamber 32. The outer shell 164 for the emitter arm 144includes stanchions 170 which secure the LED circuit board 148 in theproper position on the emitter arm 144. Similarly, the outer shell 166for the detector arm 146 includes stanchions 172 which secure thedetector circuit board 152 in the proper position.

The transmitter and processor circuit board 150 is contained within acompartment 174 in the emitter arm 144 defined by the inner housingframe 162 and the emitter arm shell 164. The receiver and communicationscircuit board 154 and the power supply board 156 are located in acompartment 176 defined by the inner housing frame 162 and the detectorarm shell 166. In order to avoid vibration damage to the boards 150, 154and 156 (e.g., due to sonic welding of the housing components), it hasbeen found desirable that the board 150 in the compartment 174 andboards 154 and 156 in the compartment 176 not be mounted directly to thehousing frame or outer shells. The power supply board 156 is physicallymounted to the receiver and communications circuit board 154. One end ofthe receiver and communications circuit board 154 is supported by theflexible ribbon cables 160, and the other end is supported by the moldedrubber strain relief for the serial cable (e.g. USB) 158. The receiverand communications board 154 is also connected via jumper 184 to thedetector board 152. This mounting arrangement enables the boards 154 and156 to float in the housing compartment 176 and isolate the boards frompotentially damaging vibrations. Components on the detector board 152 aswell as the LED board 148 are encapsulated within epoxy to secure thecomponents to the boards 152, 148 and protect the components fromvibration damage. The transmitter and processor circuit board 150 isheld by the flexible ribbon cable 160 and also jumper 180. Similarly,this mounting arrangement enables the board 150 to float in the housingcompartment 174 in the emitter arm 144 and isolate the board 150 frompotentially damaging vibrations.

It will be appreciated that the shrouds depicted above in FIGS. 3-6 aremost advantageous in extreme situations, such as when a patient has verylow oxygen levels in venous blood. Thus, although FIGS. 3-6 depictshrouds for blocking ambient light, an alternative embodiment of thesensor clip assembly 34 depicted in FIGS. 3-6 may not include theshrouds for blocking ambient light as described above. Furthermore, itwill be appreciated that the embodiment of the sensor clip assembly 34depicted by FIGS. 3-6 is merely exemplary and that one skilled in theart would be able to modify the configuration of various componentswithout departing from the inventive principles described herein.

Turning now to FIG. 7, a general process for initializing and performingblood monitoring is depicted. At step 701, a user first powers themonitoring system on, and, at step 703, the system is initialized,calibration parameters are loaded, control registers are configured, andsystem timers are started. The calibration parameters are initiallydetermined after a sensor clip assembly is manufactured, and may beupdated in the field when appropriate.

Calibrations at the factory are initially completed by measuringabsorptive filters constructed inside a blood chamber (“factorycalibration filters”). These factory calibration filters are constructedof stable, light passing materials and built to provide reference pointsin absorption that correlate to actual transmission ratios found inblood. While a single factory calibration filter can be used, thepreferred method is to use at least two factory calibration filters withdifferent transmissive light values per wavelength such that calibrationslopes (gains) and intercepts (offsets) can be established for eachwavelength. These slopes and intercepts are stored in non-volatilememory (either in the sensor clip assembly 34 or in the external hostdevice) and used in measurements to ensure the signals are accuratelyinterpreted into blood values. It is common to verify that thecalibrations are accurate by circulating human blood in a closed circuitand measuring the blood against a known measurement device such as acell counter. This is done at different hematocrit and oxygen levels tovalidate the calibration of the sensor clip assembly 34.

After the sensor clip assembly 34 is calibrated, it is assigned a uniqueverification filter that may be attached to the data cable or to anexternal host device that is interfacing with the sensor clip assembly.It is common practice that at least monthly, the user places the sensorclip on the paired unique verification filter and verifies that thesensor clip assembly 34 reads the same values from the filter as when itwas calibrated. If the values fall within limits of the originalmeasurement plus or minus a prescribed offset, then the sensor clipassembly 34 “passes” the verification test and is allowed to continue tofunction. If the measurements on the filter fall outside the limits,then the device is taken out of service.

After a single verification failure, the user should clean the surfacesof the sensor clip assembly 34 and ensure the sensor clip assembly 34 isseated properly on the verification filter. Verification is attempted asecond time. If it the device again fails, the option to field calibrateis presented to the user. With the sensor clip assembly 34 in place onthe verification filter, an algorithm correlates the current value ofmeasurement to that when the device was calibrated. New correctionvalues are calculated and implemented in the software. If the sensorclip assembly 34 is too far out of the boundaries established forreliable field calibration, the device remains disabled and should bereplaced. If the device successfully recalibrates, an additionalverification test is made. Passing of the verification test places theunit back in service.

After the system is ready and a patient has begun hemodialysistreatment, raw analog data is collected by the sensor clip assembly atstep 707. The signals received are in response to illumination of theblood by the sequentially powered LEDs. This raw analog data includesraw analog current signals received at the photodetectors based onoxygen, hematocrit, and water-sensitive LED frequencies as well astemperature readings. These raw analog current signals are convertedinto the voltage domain by transimpedence amplifiers, processed by asignal conditioning circuit, and then digitized by an A-to-D converter.

At step 709, the sensor clip assembly 34 calculates the hematocrit,oxygen saturation, and change in blood volume associated with bloodpassing through the blood chamber 32 to which the sensor clip assembly34 is attached based on the raw data and calibration parameters, using aratiometric model, substantially as disclosed in U.S. Pat. No. 5,372,136entitled “System and Method for Non-Invasive Hematocrit Monitoring”,issued on Dec. 13, 1999 and assigned to the assignee of the presentapplication, which is incorporated by reference herein in its entirety.The intensity of the received light at each of the various wavelengthsis reduced by attenuation and scattering from the fixed intensity of thevisible and infrared light emitted from each of the LED emitters. Beer'sLaw, for each wavelength of light, describes attenuation and scatteringas follows:i _(n) =I _(0-n) *e ^(−ε) ^(p) ^(X) ^(p) ^(d) ^(pt) *e ^(−ε) ^(b) ^(X)^(b) ^(d) ^(b) *e ^(−ε) ^(p) ^(X) ^(p) ^(d) ^(pr)   Eq. (1)where i_(n)=received light intensity at wavelength n after attenuationand scattering; I_(o-n)=transmitted light intensity at wavelength nincident to the measured medium; e=the natural exponential term; ε=theextinction coefficient for the measured medium (p—blood chamberpolycarbonate, b—blood); X=the molar concentration of the measuredmedium (p—blood chamber polycarbonate, b—blood); and d=the distancethrough the measured medium (pt—transmitting blood chamberpolycarbonate, b—blood, pr—receiving blood chamber polycarbonate).

Since the properties of the polycarbonate blood chamber do not change,the first and third exponential terms in the above Eq. (1) are constantsfor each wavelength. Mathematically, then these constant terms aremultiplicative with the initial constant term Io-n which represents thefixed intensity of the radiation transmitted from the respective LEDemitter. For simplification purposes, Eq. (1) can be rewritten in thefollowing form using bulk extinction coefficients and a modified initialconstant I′_(o-n), as follows:i _(n) =I′ _(o-n) *e ^(−α) ^(b) ^(d) ^(b)   Eq. (2)where i_(n)=received light intensity at wavelength “n” after attenuationand scattering as though the detector were at the receive bloodboundary; α=the bulk extinction coefficient (α_(b)=ε_(b)X_(b)) andI′_(o-n)=the equivalent transmitted light intensity at wavelength n asif applied to the transmit blood boundary accounting for losses throughthe blood chamber. Note that the term I′_(o-n) is the light intensityincident on the blood with the blood chamber losses included.

Using the approach defined in Eq. (2) above, the 810 nm wavelength whichis isobestic for red blood cells and the 1300 nm wavelength which isisobestic for water can be used to determine the patient's hematocrit.The ratio of the normalized amplitudes of the measured intensity atthese two wavelengths produces the ratio of the composite extinctionvalues α for the red blood cells and the water constituents in the bloodchamber, respectively. A mathematical function then defines the measuredHCT value:

$\begin{matrix}{{HCT} = {f\left\lbrack \frac{\ln\left( \frac{i_{810}}{I_{0 - 810}} \right)}{\ln\left( \frac{i_{1300}}{I_{0 - 1300}} \right)} \right\rbrack}} & {{Eq}.\mspace{14mu}(3)}\end{matrix}$where i₈₁₀ is the light intensity of the photo receiver at 810 nm, i₁₃₀₀is the infrared intensity of the photodetector at 1300 nm and I₀₋₈₁₀ andI₀₋₁₃₀₀ are constants representing the intensity incident on the bloodaccounting for losses through the blood chamber. The above equationholds true assuming that the flow of blood through the blood chamber 32is in steady state, i.e. steady pressure and steady flow rate.

The preferred function f[ ] is a second order polynomial having thefollowing form:

$\begin{matrix}{{HCT} = {{f\left\lbrack \frac{\ln\left( \frac{i_{810}}{I_{0 - 810}} \right)}{\ln\left( \frac{i_{1300}}{I_{0 - 1300}} \right)} \right\rbrack} = {{A\left\lbrack \frac{\ln\left( \frac{i_{810}}{I_{0 - 810}} \right)}{\ln\left( \frac{i_{1300}}{I_{0 - 1300}} \right)} \right\rbrack}^{2} + {B\left\lbrack \frac{\ln\left( \frac{i_{810}}{I_{0 - 810}} \right)}{\ln\left( \frac{i_{1300}}{I_{0 - 1300}} \right)} \right\rbrack} + {C.}}}} & {{Eq}.\mspace{14mu}(4)}\end{matrix}$

A second order polynomial is normally adequate as long as the infraredradiation incident at the first and second wavelengths is substantiallyisobestic.

The oxygen saturation level, or the oxygenated hemoglobin level, isdetermined with a ratiometric model having the following form:

$\begin{matrix}{{SAT} = {g\left\lbrack \frac{\ln\left( \frac{i_{660}}{I_{0 - 660}} \right)}{\ln\left( \frac{i_{810}}{I_{0 - 810}} \right)} \right\rbrack}} & {{Eq}.\mspace{14mu}(5)}\end{matrix}$where i₆₆₀ is the light intensity of the photo receiver at 660 nm, i₈₁₀is the intensity of the photodetector at 810 nm and I₀₋₆₆₀ and I₀₋₈₁₀are constants representing the intensity incident on the bloodaccounting for losses through the blood chamber. The function g[ ] is amathematical function determined based on experimental data to yield theoxygen saturation level, again preferably a second order polynomial. Itmay be useful to use a pair of second order polynomials depending on thehematocrit value or a separate 810 nm calibration for oxygen andhematocrit. Similar as in the case with the calculation for hematocrit,errors in the oxygen saturation value SAT can occur if there are errorsin the measured intensity of the light at either the 660 nm or 810 nmwavelengths.

After these calculations are performed, at step 711, the resulting datais output by the sensor clip assembly through a serial port (e.g., suchas a USB connector) to a device capable of displaying the data (e.g., acomputer with a monitor). These steps of collecting raw data,calculating hematocrit, oxygen saturation, and blood volume change, andoutputting the data through the serial port continue to be performed(i.e., the process loops back to node A at step 705) until the system ispowered off at step 713. It will be appreciated that these steps may beoccurring simultaneously (e.g., while certain raw data is being used incalculations or processed data is being output through the serial port,other raw data is being collected at the same time).

As mentioned above, the collection of raw data, the calculation ofhematocrit, oxygen saturation, and blood volume change, and theoutputting of data through a serial port are all performed by componentsof the sensor clip assembly 34. Providing this functionality at thesensor clip assembly 34 advantageously allows analog signal data fromthe photodetectors to be collected and converted into digital signalswithout significant transmission losses, which in turn reduces theamount of noise present in output data that is ultimately displayed.Additionally, converting data into digital from within the sensor clipassembly 34 reduces the transmission distance of the analog signals,which reduces the amount of noise introduced by the analog transmissionand allows suitable signal-to-noise ratios to be achieved at lowertransmitter power. Thus, the system is able to drive the LED emitterswith lower electrical currents, which lowers heat generation and extendsthe useful life of the LEDs, as well as the time needed betweencalibrations.

Turning now to FIG. 8, the general process of FIG. 7 will be describedin greater detail with respect to the components of a sensor clipassembly 34. FIG. 8 depicts the communication of electrical signals inthe context of the sensor clip assembly 34 (see FIGS. 5 and 6). Thereare a plurality of electrical connections 180 between the transmitterand processor circuit board 150 and the LED circuit board 148. Thetransmitter and processor circuit board 150 includes a microcontroller182, which among other tasks controls the input current to the LEDemitters on the LED board 148 via the conductors 180. As mentioned, theLED circuit board 148 preferably includes an LED emitting red light atabout 660 nm, an LED emitting infrared light at about 810 nm and anotherLED emitting infrared light at about 1300 nm. The microcontroller 182preferably includes a built-in A-D convertor. The microcontroller 182controls the current output to the LEDs, preferably so that each LEDoutputs a calibrated known intensity at the respective wavelength. Asmentioned above, the microcontroller 182 should be calibrated initiallyand re-calibrated when necessary to account for differences in outputefficiency of the LEDs for each clip assembly. Alternatively, in afurther embodiment, because the sensor clip assembly is relativelyinexpensive to manufacture, the sensor clip assembly is simply replacedonce the clip assembly falls out of calibration.

Dashed line 178 depicts visible and/or infrared light being transmittedfrom an LED on the LED circuit board 148 to one of the photodetectors onthe detector circuit board 152. The detector board 152 includes at leastone silicon photodetector and at least one indium gallium arsenidephotodetector. The microcontroller 182 implements a multiplexing routineso that LED emission is active and correlated to its respective receivedsignal through the photo diodes for visible and infrared light. Oneexample of multiplexing is the time based switching of each LED andmatching detector for unique successive time periods resulting in timeperiod measurements unique to each wavelength. This time based method iscalled commutation. A plurality of conductors connects the detectorboard 152 to the receiver and communications circuit board 154. Theconductors 184 include paths to ground, as well as electricalconnections to the anode and cathode of the silicon diodephotodetector(s) and an electrical connection to the anode and cathodeof the indium gallium arsenide diode photodetector(s).

The signals from the photodetectors are normally relatively weak (in theμA range) with a poor signal to noise ratio. The receiver andcommunications board 154 includes transimpedance amplifiers 186 thatconvert the analog current signals (μA) from the silicon and indiumgallium arsenide photodetectors into analog voltage signals (mV). Theanalog voltage signals from the transimpedance amplifier 186 aretransmitted to digital trim pots 188. Conductors 194 transmit timingsignals from the microcontroller 182 to control the synchronization ofthe trim pots 188 in order to ensure that proper time-based commutationoccurs. The time-commutated, voltages signals from the trim pots 188 aretransmitted to a summing junction. The composite time-commutated,voltage signal from the summing junction is then processed throughsignal filtering hardware 190 to strip noise from the analog voltagesignal. The cleaned analog signal is then separated by themicrocontroller 182 through line 192 to the built-in A-D converter whereeach signal is measured separately. These de-commutated signalsrepresent the intensity of the visible and infrared light at therespective wavelength 660 nm, 810 nm, or 1300 nm as appropriate asdepending on the time in the de-commutation process.

The microcontroller 182 is programmed with the calibrated, ratiometricmodel (substantially as described in U.S. Pat. No. 5,372,136 mentionedabove) to calculate the patient's hematocrit. It is also preferablyprogrammed with a calibrated, ratiometric model to calculate thepatient's oxygen saturation level. The HCT and SAT values are based onthe detected signals from the silicon and indium gallium arsenidedetectors that are filtered, de-commutated and calculated by themicrocontroller 182. The ratiometric model for calculating the HCT is ofthe form of Eq. (3) referred to above, and is preferably a second orderpolynomial having a form as described in the above Eq. (4). Theratiometric model for determining the oxygen saturation level (SAT) isof the form of Eq. (5) above, and preferably is in the form of a secondorder polynomial as well.

The calculated values for HCT and SAT are output as digital signals bythe microcontroller 182 via conductor 196 and are transmitted to aserial communications chip 198 on the receiver and communications board154. The serial communications chip converts the digital signals fromthe microcontroller 182 into data signals that are transmitted via lines200 to the serial cable 158. It is preferred to transmit the datasignals by a USB cable using conventional USB protocol.

The data transmitted via the serial (e.g. USB) cable 158 preferablyincludes systems status information as well as the real-time HCT and SATinformation, and also preferably real-time hemoglobin and change inblood volume information that can be calculated from the HCTinformation. Other data calculated by the microcontroller 182 can alsobe transmitted via the serial cable 158 in a similar manner. Desirably,a USB cable transmits the data to another piece of equipment, such as astand-alone or networked personal computer, that can accept the USBcable receptacle and data as is known in the art. An exemplary formatfor an output data stream with a corresponding table, Table I, isprovided below:

<STX>D c hh.h oo.o ssssssss xxxx <CR><LF>

TABLE I Exemplary Output Data Stream Character/Field Description <STX>0x02, Start of text control character D ASCII ‘D’ C ASCII integerrepresentation of the counter hh.h ASCII decimal representation of theHematocrit oo.o ASCII decimal representation of the Oxygen SaturationSsssssss ASCII hex representation of the 32 bit status bits. Xxxx ASCIIhex representation of the 16 bit CRC. The CRC generation includes thedata starting with the first character following the leading <STX>character up to and including the space “ ” character preceding the CRCvalue. The CRC calculation does not include the <STX>, the CRC nor the<CR><LF> characters. <CR> 0x0D, Carriage return character <LF> 0x0A,Line feed characterAlthough not depicted in Table I, it will be appreciated that an errordetection protocol such as a checksum may be included in the output datastream.

Instructions to the sensor clip assembly 34 can be transmitted fromconnected equipment (e.g., a computer) over the USB cable 158, throughthe USB communications chip 198 on the receiver and communications board154 and via conductor 202 to control the microcontroller 182 as well.Table II below provides an exemplary set of commands and correspondingdescriptions that may be used:

TABLE II Exemplary User Command Set Command Description a Verifyaccuracy f Perform field calibration o Set the output mode flag. Theoutput mode flag allows the operator to customize the normal mode outputdata. Regardless of the flag setting the Hct, Sat, and Status willalways be output. <o NN> where NN range “00”- “FF”. The bits are definedas follows: Bit 0 = Include Unit ID Bit 1 = 0 = Counter roll over @ 10,1 = Continuous counter Bit 2 = Include raw Hct value Bit 3 = Include LEDvoltages Bit 4 = Include Temperature and Reference voltages Bit 5 =Include 800% T (Hct value) Bit 6 = Include % T values (Overrides Bit 5)Bit 7 = Disable input ‘echo’ r System reset rv Generate CLM “rvt” styleoutput. s Set sample rate. <s n> where: n = “1” ( One sample per second)Default n = “2” (Two samples per second) n = “A” (Ten samples persecond) n = “B” (One sample every two seconds) t Enable/Disable dataoutput <t 1> enable, <t 0> disable, <t> toggle u Get unit id. x Set LEDsoff y Set LEDs on z Set LED sleep mode. Setting the LED's on cancelssleep modeAlthough not depicted in Table II, it will be appreciated that an errordetection protocol such as a checksum may be included with the usercommands.

The USB cable 158 provides 5V USB power to the power supply board 156.The power supply board 156 conditions the power from the USB port, andisolates the electrical components on the sensor clip assembly 34 fromdirect connection to the USB power which may not be smooth enough forreliable operation of the sensor clip assembly. The power supply board156 regenerates quiet and precise 5V and 3.3V power in order tofacilitate reliable operation of the LED emitter and detector pairs aswell as the other electronic components on the sensor clip assembly 34.The power supply board 156 uses switching regulators to convert betweenthe 5V and the 3.3V power signals as needed. It has been found that theswitching regulators are quite efficient and do not generate asignificant heat load.

FIG. 9 provides a functional block diagram of the sensor clip assembly34 described above with respect to FIG. 8. The microcontroller 182generates timing signals 903, 905 for both the transmitter and thereceiver sections of the sensor clip assembly 34. The transmitter keyseach wavelength of light in turn to illuminate the blood under test, andthe resulting amplitude of each signal is measured by a correspondingdetector diode. The measured amplitudes are then used to calculate theblood parameters. This method of signaling is termed time-domainmultiplexing. While time-domain multiplexing in an exemplary embodimentis explained with more detail herein, it will be appreciated that othermethods of multiplexing are possible. It will further be appreciatedthat the microcontroller 182 includes a tangible, non-transientcomputer-readable medium (such as flash memory, RAM, EEPROM, etc.) andthat the operations performed by the microcontroller are pursuant to aprocessor executing computer-executable instructions stored on thecomputer-readable medium.

In this example, the LED emitter with a wavelength sensitive to oxygen941 is keyed on first. On the receiver side the silicon photodetector911 is used during this time interval. A gain of thedigitally-controlled trimpot resistor 188 for the InGaAs channel is setto zero and the appropriate gain is set with a digitally-controlledtrimpot resistor for the silicon channel. The signal is then filtered toremove noise and fed to a detector circuit that generates a DirectCurrent (DC) voltage level sufficiently high for an Analog to DigitalConverter (ADC) 931 in the microcontroller 182 to measure (the filtercircuit and detector circuit are depicted as a single block 921). Theresolution of the signal can be controlled by software feedback to thedigital trimpot resistor such that if too few bits on the ADC areactivated, the signal can be increased in level for the nextmeasurement. Because the receiver side is synchronized to thetransmitter signal by the microcontroller 182 via timing signals 903,905, measurements are only made when the transmitters are active. Thisadvantageously reduces the processing load on the microcontroller 182.

After a first measurement is complete, the LED emitter with a wavelengthsensitive to oxygen 941 is turned off for a period of time called a“guard band.” This time allows for the receiver circuitry to settle backto the non-signal state and prevents residual signal from overlappinginto a new measurement due to capacitor delays or ringing. After theguard band time, the next LED emitter, with a wavelength sensitive tohemoglobin 943, is turned on. The silicon detector 911 is again used asdescribed above to perform the measurement.

When this hemoglobin-related measurement is complete, the LED emitter943 is turned off and another guard band time elapses. Then the LEDemitter that is sensitive to water concentration 945 is turned on. ThisLED emitter 945 generates a wavelength that corresponds to the InGaAsphotodetector 913. During this measurement, the gain of the silicontrimpot 188 is set to zero and the gain of the InGaAs trimpot 188 is setup to the required value to facilitate a DC measurement proportional tothat channel's amplitude.

As described above, the ratio of the oxygen measurements to thehemoglobin measurements allows calculation of the oxygen saturation ofthe blood as a percentage, and the ratio of the hemoglobin measurementsto water concentration measurements allows calculation of the percentageof red cells per unit blood volume (i.e., “Hematocrit”). Thesecalculations are performed by the microcontroller 182, transmittedthrough a serial communications chip (e.g., a level convertercommercially available from Future Technology Devices International,Ltd., an “FTDI level converter”) 198, and output to an external hostdevice through a serial communications cable such as the USB cable 158.It will be appreciated that the external host device may be aconventional personal computer with appropriate software, or other typeof device incorporating USB hosting capabilities such as a PDA (personaldigital assistant) or similar type of device capable of executingsoftware for processing a data stream output from the sensor clipassembly 34.

The operation of the microcontroller 182 with respect to synchronizingthe operation of the LED emitters and photodetectors is described infurther detail with respect to FIG. 10. “Wavelength λ-1” corresponds tothe LED emitter that generates wavelength sensitive to oxygen,“Wavelength λ-2” corresponds to the LED emitter that generateswavelength sensitive to hemoglobin, and “Wavelength λ-3” corresponds tothe LED emitter that generates wavelength sensitive to waterconcentration. As described above with respect to FIG. 9, a first LEDemitter 941 corresponding to “Wavelength λ-1” is turned on, and theSilicon channel is simultaneously activated by appropriately adjustingthe gains of the digital trimpots 188 (see Trace A and Trace B). Then,the first LED emitter is turned off and the Silicon channel isdeactivated, and after a “guard band”, a second LED emittercorresponding to “Wavelength λ-2” is turned on while the Silicon channelis simultaneously activated. Similarly, after the second LED emitter isturned off and the Silicon channel is deactivated, and after another“guard band,” a third LED emitter corresponding to “Wavelength λ-3” isturned on while the InGaAs channel is activated. FIG. 10 further depictsdata burst control timing in trace C. Due to the sensitivity of theanalog signal conditioning circuits, it is advantageous to transmitdigital data through the output port during the guard band such that thetransmission of digital data does not interfere with the acquisition andconditioning of the raw analog signals. FIG. 10 further depicts that theLED current needed to operate each LED emitter may be different (as seenin Trace B). This process of time-domain multiplexing of the LEDemitters and receiving channels is repeated throughout the course ofdata acquisition.

In a further embodiment, as depicted in the functional block diagram ofFIG. 11, the calculation of the properties of blood and generation ofcontrol signals instead take place at an external device, such as anetworked or stand-alone personal computer, and the sensor clip assembly34 is responsible for acquisition of raw digital data (i.e., raw analogdata generated by the photodetectors that has been conditioned andconverted to digital format). The sensor clip assembly 34 of thisembodiment includes LED emitters 941, 943, 945, photodetectors 911, 913,Silicon and InGaAs channels, a signal conditioning circuit 970(including amplifiers 186, digital trimpots 188, and filter/detectioncircuit block 921 as described above with respect to FIG. 9), ananalog-to-digital converter 931, and a digital signaling cable 980connected to an external host device. The sensor clip assembly 34receives receiver control signals and outputs data over connectors 960,and receives emitter control signals over connectors 961. The sensorclip assembly 34 also receives power from the digital signaling cable980. It will be appreciated that the sensor clip assembly 34 of thisembodiment also reduces the transmission distance of the analog signals,which reduces the amount of noise introduced by the analog transmissionand allows suitable signal-to-noise ratios to be achieved at lower poweras described above.

Turning to a further embodiment of the sensor clip assembly 34 depictedin FIGS. 8-9, the sensor clip assembly 34 is connected via the USB cable158 to a computer programmed with software to receive data from thesensor clip assembly 34 and to display it on a screen. Because the rawdata is collected, converted to digital signals, and calculated at thesensor clip assembly 34, the computer is not subject to calibrationrequirements and need not include ratiometric calculation capabilities.FIG. 12 provides an exemplary demo software interface that allows theuser to view data received from the sensor clip assembly 34, as well asto issue commands to the sensor clip assembly 34 (pursuant tocomputer-executable instructions being carried out by a softwareapplication being run by the computer). It will be appreciated that thedemo software interface of FIG. 12 is merely exemplary and that theconfiguration and types of information and/or options presented to theuser may be varied. For example, commercial software applicationsintended for a commercial user may include less information since acommercial user may not need to use the detailed status information orview the received data stream.

The “COM Port” section 1001 of the interface allows the user to select aCOM number that corresponds to the sensor clip assembly 34 that the userwants to interact with. A single computer having multiple USB ports canaccommodate more than one of the sensor clip assemblies 34, and thusmultiple sensor clip assemblies may be connected to the computer at thesame time. It will be appreciated that in a further embodiment, thesoftware interface may allow information received from multiple sensorclip assemblies to be viewed simultaneously, as well as allowing for thesimultaneous control of multiple sensor clip assemblies. In a furtherembodiment, the computer to which the sensor clip assembly 34 isconnected may be wirelessly connected to a host computer that executesthe software application to control one or more sensor clip assemblies34 remotely over a wireless connection.

The “Log File” section 1003 of the interface allows the user to storedata received from the sensor clip assembly 34 in a log file at auser-designated (or automatically generated) location. The user cantoggle this logging function on or off by checking the box next to theword “Log.”

The “Input Data” section 1005 of the interface displays incoming datafrom the sensor clip assemblies 34 in an exemplary format similar to theformat described in Table I above. The “Parsed Data” section 1007 of theinterface shows a unit ID and filter ID corresponding to the sensor clipassembly 34 from which data is being received, as well as “Count,”“Hct,” “Sat,” and “Status” information, corresponding to a count value,a Hematocrit value, an oxygen saturation value, and status information,respectively. The “Count” value is an approximate time counter. The usercan check the “Count Flag” box to cause the count value to increment atone second intervals indefinitely. If the “Count Flag” box is notchecked, the count value will roll over after it reaches a value of 9.The “Status Bits” section 1009 of the interface shows whether certainitems are set or cleared based on the “Status” information received fromthe sensor clip assembly 34.

The “Control Functions” section 1011 of the interface provides a fewcommands that the user can issue to the sensor clip assembly 34. The“Verify” button provides the user with an option to verify orre-calibrate the sensor clip assembly 34. If the user chooses to verifythat the device is still operating within a proper range, the sensorclip assembly 34 must be attached to the verification filter uniquelycorresponding to that sensor clip assembly 34 in order for theverification to be accurate. As described above with respect to FIG. 7,if the verify function fails twice, then the system is out of serviceand eligible for field recalibration. Before the system can berecalibrated, the screen will display a prompt asking the user toconfirm the ID of the verification filter to which the sensor clipassembly is fastened. If the sensor clip assembly is fastened to anincorrect verification filter, the recalibration cannot be performed.

The “Turn LEDs Off” button turns the LED emitters off (and changes to a“Turn LEDs On” button after the user has chosen to turn the LEDs off).Manually turning off all the LEDs when the sensor clip assembly 34 isnot in use lengthens the service life of the sensor clip assembly 34.The “Reset” button resets the sensor clip assembly 34 (i.e., to step 701of the process depicted by FIG. 7) without resetting the port connection(i.e., a reset does not re-enumerate the sensor clip assembly 34 on theUSB port or otherwise affect the USB connection).

The “Patient Run” section 1013 of the interface provides the user withthe option to “Start Run,” which causes the application to begin loggingpercent blood volume change (% BV Change), hematocrit (Hct), and oxygensaturation (Sat) values once a minute to a delimited text file which maybe manipulated, for example, by a spreadsheet or database application(distinct from the log file shown in the “Log File” section 1003 of theinterface). The name of the file is shown in the window next to the“Start Run” or “End Run” button. FIG. 12 shows a patient run that iscurrently in progress, and thus the “Start Run” button had previouslybeen pressed and an “End Run” button is currently displayed to the user.The graphs in the “Patient Run” section 1013 of the display are agraphical representation of the data stored in the text file, and allowthe user to visually monitor the % BV, Hct, and Sat values over time.

In the “Exit” section 1015 of the interface, the user can exit thesoftware application by clicking on the “Exit” button.

FIGS. 13A-F depict an exemplary user interface of a software applicationfor commercial users of a sensor clip assembly 34 pertaining toverification and recalibration. FIG. 13A shows the user interfacepresented to the user after the user has chosen to verify the accuracyof a sensor clip assembly 34 and the verification has failed (i.e., thereadings of the sensor clip assembly 34 are not within a predeterminedtolerance range). As shown in FIG. 13A, the user is notified that theaccuracy verification has failed and is advised to ensure that thesensor clip assembly is properly attached to the verification filter andto ensure that the verification filter is clean. After the user presses“OK” and tries to verify the accuracy of the sensor clip assembly 34again, and the verification again fails, the user is presented with theinterface shown in FIG. 13B, which notifies the user that theverification has failed and gives the user the option to attempt a fieldcalibration, If the user selects “Yes” in the interface shown in FIG.13B, the user is taken to the screen shown in FIG. 13C, which notifiesthe user that the user will need to enter the identification codecorresponding to the verification filter (which is a serial number thatcan be obtained from the verification filter itself). After the userpresses “OK,” the user is prompted to enter the identification code asshown in FIG. 13D The software then compares the entered identificationcode with an identification code corresponding to the sensor clipassembly 34 obtained from the sensor clip assembly 34 or previouslystored in the external host device, and if a match is found, the user isnotified that the entered identification code has been accepted as shownin FIG. 13E. After the user presses “OK” in the screen shown in FIG.13E, the sensor clip assembly 34 is field recalibrated and the softwareonce again tries to verify the accuracy of the sensor clip assembly 34.If this verification after field recalibration fails, the user will benotified that the sensor clip assembly 34 (called a “Crit-Line sensorclip” in this example) needs to be replaced, as shown in FIG. 13F. Theuser may attempt additional field recalibrations.

It will be appreciated that, with respect to the embodiment of thesensor clip assembly depicted in FIG. 11, the software applicationsdescribed above with respect to FIGS. 12 and 13A-13F may be modified toreceive raw digital data from the sensor clip assembly 34, performratiometric calculations based on the received raw digital data, displaysimilar results to a user, and to be verified and recalibrated asdescribed above.

While the embodiments described above have focused on the collection ofdata regarding percent blood volume change, hematocrit values, andoxygen saturation values, it will be appreciated that other types of LEDemitters paired with the same or other types of photodetector diodes maybe used to measure other types of parameters.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate, and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

The invention claimed is:
 1. A system for optically monitoring blood,the system comprising: a sensor clip assembly, comprising: a housing; anemitter within the housing, configured to emit light through averification filter to which the sensor clip assembly is fastened,wherein the verification filter is associated with the sensor clipassembly; a photodetector within the housing, configured to detect lightfrom the emitter that has passed through the verification filter; and amicrocontroller within the housing, configured to verify accuracy of thesensor clip assembly based on whether readings corresponding to lightdetected by the photodetector are within a predetermined tolerancerange; and  a host device, in communication with the sensor clipassembly, wherein the host device comprises a display configured todisplay a result of the verification to a user.
 2. The system accordingto claim 1, wherein the display is configured to present an option toperform field recalibration for the sensor clip assembly after a numberof failed verifications.
 3. The system according to claim 2, wherein thehost device is further configured to receive a user input to performfield recalibration for the sensor clip assembly.
 4. The systemaccording to claim 3, wherein the host device is further configured torequest user input of a verification filter identification codecorresponding to the verification filter in response to receiving theuser input to perform field recalibration for the sensor clip assembly.5. The system according to claim 4, wherein the microcontroller isconfigured to perform field recalibration for the sensor clip assemblyin response to a correct verification filter identification code beingreceived.
 6. The system according to claim 5, wherein performing fieldrecalibration comprises calculating new correction values to be appliedfor the sensor clip assembly.
 7. The system according to claim 4,wherein the microcontroller is configured to attempt field recalibrationfor the sensor clip assembly in response to a correct verificationfilter identification code being received, and the display is configuredto notify the user that field recalibration failed if themicrocontroller was not able to perform field recalibration.
 8. A methodfor operating a sensor clip assembly for optically monitoring blood, themethod comprising: fastening the sensor clip assembly to a verificationfilter, wherein the verification filter is associated with the sensorclip assembly; emitting, by an emitter of the sensor clip assembly,light through the verification filter to obtain readings correspondingto light detected by a photodetector of the sensor clip assembly;verifying, by a microcontroller within a housing of the sensor clipassembly, the obtained readings are within a predetermined tolerancerange; and displaying, via a host device in communication with thesensor clip assembly, a result of the verification to a user.
 9. Themethod according to claim 8, further comprising: presenting, via thehost device, an option to perform field recalibration for the sensorclip assembly after a number of failed verifications.
 10. The methodaccording to claim 9, further comprising: receiving, via the hostdevice, a user input to perform field recalibration for the sensor clipassembly.
 11. The method according to claim 10, further comprising:requesting, via the host device, user input of a verification filteridentification code corresponding to the verification filter in responseto receiving the user input to perform field recalibration for thesensor clip assembly.
 12. The method according to claim 11, furthercomprising: performing, by the microcontroller, field recalibration forthe sensor clip assembly in response to a correct verification filteridentification code being received.
 13. The method according to claim12, wherein performing field recalibration comprises calculating newcorrection values to be applied for the sensor clip assembly.
 14. Themethod according to claim 11, further comprising: attempting, by themicrocontroller, field recalibration for the sensor clip assembly inresponse to a correct verification filter identification code beingreceived; and notifying, via the host device, that field recalibrationfailed in response to the microcontroller not being able to performfield recalibration.
 15. A sensor clip assembly for optically monitoringblood, the sensor clip assembly comprising: a housing; an emitter withinthe housing, configured to emit light through a verification filter towhich the sensor clip assembly is fastened, wherein the verificationfilter is associated with the sensor clip assembly; a photodetectorwithin the housing, configured to detect light from the emitter that haspassed through the verification filter; and a microcontroller within thehousing, configured to verify accuracy of the sensor clip assembly basedon whether readings corresponding to light detected by the photodetectorare within a predetermined tolerance range.
 16. The sensor clip assemblyaccording to claim 15, wherein the microcontroller is further configuredto perform field recalibration for the sensor clip assembly.
 17. Thesensor clip assembly according to claim 16, wherein performing fieldrecalibration comprises calculating new correction values to be appliedfor the sensor clip assembly.
 18. The sensor clip assembly according toclaim 15, wherein the microcontroller is further configured to performfield recalibration for the sensor clip assembly after a number ofverification failures.
 19. The sensor clip assembly according to claim15, wherein the microcontroller is further configured to confirm userinput of a correct verification filter identification code prior toperforming field recalibration for the sensor clip assembly.